[WinEH] Remove unreachable blocks before preparation

[WinEH] Remove unreachable blocks before preparation

We remove unreachable blocks because it is pointless to consider them
for coloring.  However, we still had stale pointers to these blocks in
some data structures after we removed them from the function.

Instead, remove the unreachable blocks before attempting to do anything
with the function.

This fixes PR25099.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249617 91177308-0d34-0410-b5e6-96231b3b80d8